The Ca channel agonist, Bay K 8644, inhibits post-rest contractions (PRCs) in ferret ventricular muscle, in addition to its effects on sarcolemmal (SL) Ca channels. As PRCs are supported primarily by sarcoplasmic reticulum (SR) Ca release, this indicates that Bay K 8644 may interfere with normal SR function (possibly by inducing a loss of SR Ca). The central issue to be addressed is whether this depressant effect of Bay K 8644 on the SR is linked to a primary action on SL Ca channels rather than a direct action on the SR (e.g. is there evidence for direct coupling between the SL and SR Ca channels?). To address this we will measure PRCs, rapid cooling contractures (RCCs) and caffeine-induced contractures to assess the rest-dependent decline in SR Ca content. The loss of SR Ca to the cytoplasm and from the cell will also assessed (e.g. with Ca-electrodes). Whole cell voltage clamp will be used to measure calcium current (Ica). To examine the possible interaction between the SL and SR Ca channels, we will also measure ryanodine and dihydropyridine binding characteristics in isolated cardiac myocytes, fractionated cardiac preparations (e.g. diads and SR) as well as in triads (and SR vesicles) from rabbit skeletal muscle. Our specific goals will be to answer the following questions: 1. Are the effects of Bay K 8644 on SR function and on SL Ica mediated by the same receptor? 2. Is Bay K 8644 itself or the Ica change which it induces responsible for rest depression? 3. Does Bay K 8644 decrease PRCs by reducing SR Ca pool size or the fractional release of SR Ca? 4. Does Bay K 8644 decrease PRCs due to either sub- or supra- optimal trigger for SR Ca release? 5. Is Na/Ca exchange involved in the enhanced diastolic efflux of SR Ca promoted by Bay K 8644? 6. Does Bay K 8644 produce an increase in SR Ca loss and/or cellular Ca efflux during rest? 7. Does Bay K 8644 alter ryanodine binding to isolated triads (or Heavy SR) from skeletal muscle? 8. Does Bay K 8644 modify the ryanodine binding in intact cardiac myocytes and subcellular fractions? This detailed analysis of the Bay K 8644 induced rest depression and interaction with ryanodine binding will provide insight into: a) the mechanism of this novel action of Bay K 8644 and b) the coupling between the SL Ca channel and the SR Ca release channel. This will provide insight into the most important remaining fundamental question about excitation- contraction coupling in cardiac (and skeletal) muscle.